For the protection of electrical equipment, it is customary to use, between the two lines of AC mains, a metal oxide varistor, especially zinc oxide, mounted in series on the one hand with a thermofusible disconnection element, and on the other hand with a gas discharge tube.
Such a device functions theoretically as follows: the gas discharge tube sustains practically the entire AC voltage of the mains. In fact, the stray capacitance of the discharge tube is of the order of a picofarad, whereas the stray capacitance of the varistor is several nanofarads to several dozen nanofarads. When an overvoltage occurs, it causes a priming of the gas discharge tube, which can only be extinguished if the so-called follow current passing through it subsequently becomes low enough. It is the resistance of the varistor which provides for the limiting of the follow current and enables the extinguishing of the gas discharge tube.
When a device for protection against overvoltages has been operated a certain number of times or in continuous manner due to a prolonged overvoltage, its components reach the end of their life. For a gas discharge tube, the end of life corresponds to a short circuiting. On the other hand, for a varistor the end of life may involve an explosion for pulsed phenomena or a strong decrease in its internal resistance (tending to a short circuit), which often may result in its catching fire. As a safety measure, the gas discharge tube may be designed so that its ability to let pass the energy pulses related to the overvoltages is less than that of the varistor. In this way, it is the gas discharge tube which is the first to reach its end of life and become short circuited.
The mains voltage is then entirely placed on the varistor, which heats up, resulting in the melting of the thermofusible element and thermal disconnection, i.e., the disabling of the protection device.
However, it is hard to make certain of the reliability of the disconnection produced by the melting of the thermofusible element.